1. Field of the Invention
The present invention relates to a fixing device for heating and thereby fixing an unfixed image such as a toner image to a record member bearing the unfixed image in an image forming apparatus such as a copying machine, a printer or the like.
2. Description of the Related Art
A fixing device in an image forming apparatus such as a printer or a copying machine generally includes a heating roller for heating and fixing an unfixed image to a record member. The record member bearing the unfixed image such as a toner image is moved between the heating roller and a backup member (generally, a pressure roller) opposed thereto, so that the unfixed image is heated and fixed by the pressure.
In many cases, the heating roller includes a heater such as a halogen lamp heater, and the roller is heated by heat radiated from the heater.
However, the heating roller containing the heater such as a halogen lamp heater as a heat source cannot rapidly heat a surface of the heating roller to a predetermined fixing temperature after start of power supply to the heater, so that a long preheating time (i.e., a warming-up time) is required before the fixing device reaches the predetermined temperature after power-on of the image forming apparatus. This prevents easy operation of the apparatus.
An example of a heating roller, which can reduce a temperature rising time for attaining the predetermined temperature, is proposed in Japanese Laid-Open Patent Publication No. 59-189381 (189381/1984). This roller includes a core roller and a resistance heating member arranged around the core roller for integral rotation. The resistance heating member is made of a substance, which generates heat when an electric current flows therethrough. The heating roller of this type has a good electrothermal converting efficiency, and can rapidly raise the surface temperature of the heating roller to a predetermined temperature after current supply to the resistance heating member, so that the preheating time of the fixing device can be reduced.
In this fixing device employing the above heating roller, the current is generally supplied to the resistance heating member via a current receiver member, which is electrically connected to the resistance heating member and rotates together with the resistance heating member, and a current supply member, which is in contact with the current receiver member. The heating roller is rotatably carried at opposite ends of the core roller by bearings, which are arranged at grounded heating roller mount units at the fixing device.
In the image forming apparatus such as a printer or a copying machine provided with the above fixing device, an openable cover is provided for accessing internal mechanisms when processing is required, e.g., for recovery from paper jamming or various kinds of maintenance. Alternatively, an upper mechanism above a sheet path may be adapted to be pivotable around a hinge arranged at the other half of the image forming apparatus. This type of apparatus is known as an image forming apparatus of a clamshell type.
In these image forming apparatuses provided with the openable cover or of the clamshell type, and particularly, in the image forming apparatus equipped with the fixing device which includes the heating roller having the resistance heating member, it is desired to eliminate a possibility of electric shock, which may be caused when an operator or a serviceman touches the fixing device or the like by opening the cover or the movable unit of the clamshell type of the image forming apparatus for recovery from paper jamming or maintenance. It is also necessary to prevent malfunction or damage of a control unit or the like, which may be caused by electrical leak from the fixing device to various portions of the image forming apparatus.
In view of the above, Japanese Laid-Open Patent Publication No. 4-305679 (305679/1992) has proposed a structure in which an electrical insulation layer interposed between a core roller and a resistance heating member is extended to appropriate positions outside current receiver members near opposite ends of a heating roller. Also, Japanese Laid-Open Patent Publication No. 59-171980 (171980/1984) has proposed a structure in which an electrical insulation layer is formed over an entire surface of a core roller forming a heating roller.
In the structure of the heating roller disclosed in Japanese Laid-Open Patent Publication No. 4-305679, the resistance heating member and the core roller is insulated from each other by the electrical insulation layer. Also, the surface of the resistance heating member is generally covered with a release layer, which also form an insulator. However, particular consideration is not given to insulation between the heating roller and a fixing device housing carrying the heating roller mount units, which may be directly touched by an operator, and particularly between the housing and a current transmission unit connected to the resistance heating member.
The reason of this is as follows. Since the electrical insulation layer insulates the resistance heating member and the core roller from each other, a designer or the like considers that current leak to the core roller is sufficiently prevented and therefore additional insulation is not required.
A current flows to the heating roller mount units in the fixing device and the housing provided with the mount units from the resistance heating member through the core roller, if the above electrical insulation layer is not employed. Even if the electrical insulation layer is provided, a current may flow to the heating roller mount units from the current receiver member, which is provided for current supply to the resistance heating member, via the core roller or directly from the current receiver member via a space.
Particularly, when the image forming apparatus is used in an atmosphere at a high humidity, it is impossible to eliminate completely the possibility of current leak through the above paths. Even if the resistance heating member is insulated from the core roller, current leak, which does not occur at a low humidity condition, may occur at a high humidity condition in the structure where appropriate insulation is not provided between the current receiver member and the core roller, and between the current receiver member and the heating roller mount units. Therefore, electric shock may occur when an operator touches the fixing device housing in the apparatus or a portion electrically connected thereto, and in other words, a possibility of electric shock cannot be eliminated completely.
In the structure disclosed in Japanese Laid-Open Patent Publication No. 59-171980, in which an electrical insulation layer 150 interposed between a resistance heating member 130 and a core roller 100 is arranged over an entire surface of the core roller 100 as shown in FIG. 9, the electrical insulation layer 150 insulates the resistance heating member 130 and the core roller 100 from each other. In this manner, the electrical insulation is provided. Also, the electrical insulation layer 150 arranged over the entire surface of the core roller 100 provides insulation between current receiver members 110 and the core roller 100. In this case, however, the electrical insulation layer 150 is present at up to ends of the core roller 100 attached to bearings, and the surface of the electrical insulation layer 150 may be damaged by a load at an inner surface of the bearing. If damaged, the heating roller cannot rotate stably, and the heating roller itself causes rotational vibration, so that wrinkles are formed at a record member during transfer between the rollers and irregular rotation of the current receiver member 110 occurs. This may result in such a state that a distance between the current receiver member 110 and a roller mount unit 170 at the housing 30 decreases, so that a leak current flows from the current receiver member 110 to the housing 30 through a space.
Generally, in the fixing device employing the heating roller having the resistance heating member described above, a current supply to the resistance heating member is controlled to set the heating roller temperature to a predetermined fixing temperature. Further, the heating roller temperature may be controlled to prevent excessive temperature rising of the heating roller for safety.
For the temperature control of the heating roller, such measures are well known and employed that a temperature sensing element of a contact type such as a thermistor is arranged for the heating roller so as to use sensed temperature information sent from the temperature sensing element for temperature control.
In the image forming apparatus such as a copying machine or a printer including the fixing device, which is provided with the heating roller having the resistance heating member as well as the temperature sensing element for sensing the temperature of the heating roller, and particularly in the image forming apparatus provided with the openable cover or of the clamshell type, there may be arranged a safety switch for immediately interrupting a current circuit to the resistance heating member when an operator or serviceman opens the cover or movable portion in view of the following possibility. When the cover or movable portion is opened, the operator or serviceman may receive an electric shock by touching the resistance heating member, or by touching a circuit or the like connected to the temperature sensing element, if the temperature sensing element is in contact with the resistance heating member via an insulation layer having an insufficient insulating performance.
However, the safety switch is located at a position remote from a power supply inlet portion, and, for example, at one side of the image forming apparatus opposite to the side of a commercial power supply inlet portion in many cases. Therefore, a power supply circuit line to the resistance heating member must be arranged via the safety switch at this remote position, which increases the line length, and therefore requires a large internal space for the long line. This impedes reduction in size of the apparatus and reduction in cost in many cases. Even if the safety switch is not remote from the power inlet portion, the power supply circuit line to the resistance heating member must be arranged through the safety switch, as is required in the above structure, so that a line or wiring for this purpose is required, which impedes reduction in size and cost of the apparatus.
In order to overcome the above problem, the safety switch may be eliminated, and appropriate safety means may be provided for avoiding electrical shock even when an operator touches a circuit or the like connected to the temperature sensing element. However, such appropriate safety means has been neither developed nor considered.
For example, as disclosed in Japanese Laid-Open Patent Publication No. 55-62474 (62474/1980), a resistance heating member may be coated with insulating elastomer having a thermal conductivity, and a temperature sensing element may be in contact with this covering layer. However, this elastomer layer alone cannot assure safety against current leak between the resistance heating member and the temperature sensing element. In a structure where the resistance heating member is arranged at an outer peripheral surface of the heating roller, the resistance heating member is coated with a release layer. This release layer alone cannot assure safety against current leak between the resistance heating member and the temperature sensing element.
In order to improve the safety, the insulating elastomer layer and the release layer may have large thicknesses, in which case responsibility of the temperature sensing element to the temperature of the resistance heating member is impaired, and thus the temperature of the heating roller cannot be controlled precisely.